Clocked digital communications systems often require timing signals which are offset in phase or delay from a known reference clock signal, either to provide an appropriate set-up or hold interval, or to compensate for propagation delay between the point of use and the location of the reference clock source. Systems relying on a single reference clock often utilize fixed or adjustable delay lines or delay circuits to generate a secondary clock signal which is time-offset from the original reference. As another example, a serial communications receiver may have a local clock synthesized from received data transitions, which may be phase-shifted an appropriate amount to allow its use in sampling the received data stream. Alternatively, systems providing a multi-phase reference clock, one example being a two-phase quadrature clock, may utilize phase interpolation techniques to generate a secondary clock signal intermediate to the two reference clock phases; in other words, having a phase offset interpolated between those of the reference clock phases.
Such phase interpolators also found extensive use in RF communications applications, as one example in producing an output signal having a particular phase relationship used to drive one element of a multi-element antenna array, such that the collection of element arrays driven by such output signals resulted in an output beam with the desired directional characteristics.
In one such application, two sinusoidal reference input signals having relative phase relationships of 90 degrees (thus commonly referred to as sine and cosine signals) are presented as inputs to the phase interpolator having an output W of:W=A*sin(ωt)+(1−A)*cos(ωt)  (Eqn. 1)
where the control input A is varied between (in this example) 0 and 1 to set the relative phase of output W as compared to reference inputs sin(ωt) and cos(ωt) Following common practice in the art, this document will utilize this well-known phase interpolator nomenclature, without implying any limitation to two clocks having that particular phase relationship, sinusoidal signals, single-quadrant versus multiple-quadrant operation, or a particular domain of applicability.